set_env: State the conclusion upfront

[ChrisDenton]

People tend to skim or skip over long explanations so we should be very upfront that set_var and remove_var are being made unsafe for a very good reason.

This is just the conclusion restated almost verbatim but earlier in the docs and separated from the explanation:

rust/library/std/src/env.rs

Lines 338 to 339 in 0c96061

	/// that no other thread will read the environment, so the only safe option is
	/// to not use `set_var` or `remove_var` in multi-threaded programs at all.

I think this may help with people who may not be entirely comfortable with #125937 being rejected.

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[jhpratt]

@bors r+ rollup

[bors]

📌 Commit 751143e has been approved by jhpratt

It is now in the queue for this repository.